不銹鋼輸水管破孔原因分析

Investigation of Leakage on the Stainless Steel Water Pipe

某公司水處理工場運送純水至生產線之8吋304不銹鋼管，於新建完成後約1個多月即發生破漏現象，該純水輸水管於配管、銲接完工後，並未施加銲後熱處理，之後曾先後以廠內循環使用之製程水與RO純水通入試車，但於實際運轉時通入RO純水即發現有多處滲水，目視該管線發生滲水之位置均位於周向與軸向銲道的位置。為了釐清不銹鋼輸水管發生破孔之原因，故進行破損分析。由草酸電解試驗結果得知，周向銲道旁的直管部分，於距銲道處約5 mm處發生明顯的敏化現象，其他區域則敏化程度較輕或未發生敏化。破損管件實際發生破孔之位置，為銲道與基材交界處，與發生敏化最嚴重之位置明顯不同，顯示破孔的形成與材質發生敏化無直接相關。另外，經破孔處型態觀察與成分分析後，得知破孔處內部並無明顯的腐蝕生成物，而僅於孔洞靠近管內壁處發現具侵蝕之Cl元素的存在。故推論造成輸水管線發生滲水的孔洞，並非由Cl-侵蝕所造成之蝕孔，而是管件進行直／彎管周向銲接組裝時，於銲道處造成的銲接缺陷或空孔，加上試水時通入製程水，水中高濃度Cl-離子侵蝕已存在於管件內部之銲接空孔，使空孔延伸至管外壁造成管內、外連通所導致。

The AISI 304 stainless steel water pipe, used for the pure water transformation from the water treatment factory to the product factory, was found to leak after the combination for a month. Post welding heat treatment (PWHT) was not applied to the pipe after welding process. In a trial run, industrial re-cyclic water and reverse osmosis (RO) pure water were subsequently injected into the pipe and then evacuated. The leakage occurred in the actual run while RO pure water injecting. After visual examination, it is found that all the leakage sites locate in the vicinity of the longitudinal and peripheral weld. The failure cause is investigated in this study. The experimental results show that the greatest degree of sensitization is revealed in 5 mm away from the peripheral weld of the straight tube. However, the leakage site of the pipe is close to the weld. The quite different location indicates that the leakage occurrence is independent to the sensitization of the pipe. In addition, aggressive Cl- is only found in the hole outer instead of inner. It demonstrates that the formation of the hole within the tube is not only resulted from pitting corrosion due to chloride ions of the industrial water. The main reason of the pipe failure is resulted from the weld decay in the peripheral welds. The minority is that the portion beside the hole due to weld decay is corroded and gets thorough the wall by high concentration aggressive chloride ions.